P461: MYELOID KINOME INHIBITOR HM43239 OVERCOMES ACQUIRED RESISTANCE IN ACUTE MYELOID LEUKEMIA MODELS
Authors: R. Bejar, S. J. Baek, N. Abbasi, A. Krasny, R. Sinha, H. Zhang, S. Howell, W. Rice
Journal: HemaSphere (2022)

Background: Oral HM43239 is in development for the treatment of acute myeloid leukemia (AML) because of its capacity to potently inhibit kinases that drive myeloid malignancies, including diverse forms of the FLT3, SYK, JAK and c-KIT kinases. Wildtype FLT3 is overexpressed in most AML patients, and approximately 30% of newly diagnosed adult AML patients harbor internal tandem duplications (ITDs) or point mutations in the tyrosine kinase domain (TKD). These mutations drive aberrant activation of downstream proliferation pathways and are associated with a high risk of relapse. Likewise, the c-KIT alternative receptor kinase, as well as the SYK and JAK1/2 intracellular kinases, mediate oncogenic signaling in AML that can promote drug resistance to certain FLT3 inhibitors. HM43239 was developed to overcome shortcomings of other FLT3 inhibitors. It inhibits a broad set of mutant and wildtype forms of FLT3, while simultaneously disrupting downstream SYK, JAK/STAT5, ERK, and other rescue signaling pathways. This rationale supports the development of HM43239. Aims: Evaluate the activity of HM43239, an orally active drug, as a FLT3-focused myeloid kinome inhibitor in human AML models. Methods: Biochemical kinase assays were performed by RBC and Carna. The effects of HM43239 on cell proliferation (IC50), growth rate (GR50) and concentration at half-maximal effect (Growth Effective Concentration; GEC50) were determined using the MTS assay with vehicle controls. Cell-based inhibition of target phosphorylation was assessed by Western blot and flow cytometric analyses. The AML cell lines tested included MV-4-11, MOLM-13, MOLM-14 and BAF3/ITD. In vivo efficacy was assessed using the MOLM-14 FLT3-Mutated xenograft model. Results: HM43239 inhibited the enzymatic activities of FLT3-WT, -ITD, and -D835Y variants with IC50 values of 1.1, 1.8 and 1.0 nM, respectively. Binding Kd values for FLT3-WT, -ITD, -D835Y, -D835H, -ITD/D835V and -ITD/F691L were 0.58, 0.37, 0.29, 0.4, 0.48, and 1.3 nM, respectively. HM43239 killed AML cells that harbor the FLT3-ITD mutation (MV-4-11, MOLM-13 and MOLM 14) with IC50, GR50 and GEC50 values ranging from 4 to 10 nM. In cell lines expressing wild type FLT3 (KG1, HEL92.1, SKM-1, and THP-1) the values for these parameters ranged from 0.05 nM to 3 µM. Western blot and flow cytometric analyses of MV-4-11, MOLM-14 cells and Ba/F cells transfected with target proteins revealed that the phosphorylation of FLT3 and SYK was reduced by 50-90% at HM43239 concentrations of 10 – 100 nM, and that these concentrations produced >80% reduction in phosphorylation of ERK and JAK/STAT5 that function downstream of FLT3. HM43239 inhibited FcγR-induced SYK and JAK/STAT5 activation in KG-1a (FLT3-WT) cells that upregulate RAS signaling which is a mechanism of acquired resistance to gilteritinib. In murine xenograft studies, HM43239 was more effective than gilteritinib in both SC and orthotopic FLT3-F691L and FLT3-ITD/F691L mutant MOLM-14 models. Summary/Conclusion: HM43239 inhibits wild type and mutant forms of FLT3 at low nM concentrations and demonstrates in vivo efficacy at doses that are well tolerated. Its ability to also inhibit SYK and, by reducing the activity of these upstream kinases, to also impair the activity of EKR1/2 and JAK/STAT5 that participate in rescue pathways, makes this a particularly interesting molecule with the potential of offsetting the development of resistance that is common with other FLT3 inhibitors. A Phase 1/2 trial of HM43239 in patients with AML is open and accruing patients (NCT03850574).